The present invention relates to a method and apparatus for raising the exhaust temperature in an internal combustion engine with a catalytic converter, and particularly to an internal combustion engine which is operated with stratified direct injection.
With stratified direct injection, non-throttled operation at low partial load or at idle causes the exhaust temperature to drop to values below 200.degree. C. When a catalytic converter is used in the exhaust line, this temperature drop cools the catalyst during prolonged operation, so that the catalyst becomes ineffective.
To increase the temperature, Japanese patent document 10054287 A proposes to adjust the ignition timing. However, the exhaust temperature in an internal combustion engine with stratified operation can be increased to only a limited extent by retarding the timing, since a longer interval between the end of injection and the ignition leads to misfires and unstable combustion.
General throttling, on the other hand, degrades efficiency because of rising charge change losses and a low degree of combustion efficiency.
One object of the invention, therefore, is to provide a method and apparatus for raising the exhaust temperature as needed, with the good efficiency of non-throttled stratified operation being used, even in short-term idle or in low partial-load operation.
This and other objects and advantages are achieved by the method and apparatus according to the invention, in which the exhaust gas temperature is monitored by means of a temperature probe or by means of an exhaust gas temperature model, and when the monitored temperature falls below a threshold value, the internal combustion engine is throttled to raise the exhaust gas temperature.
Accordingly, the exhaust temperature can be raised by throttling only if an exhaust temperature threshold is undershot.
Preferably, throttling can be time- and/or temperature-controlled so that the degree of throttling can be adapted to individual operating conditions.
According to a preferred embodiment of the invention, the catalytic converter can use precatalysts followed by temperature probes as devices to determine the exhaust temperature for example. Precatalysts in turn can be followed by a storage catalyst (NO.sub.x storage catalyst for example) which is checked during its regeneration phase by a monitor probe. The check can be performed by evaluating the lambda jump.
In order to detect the efficacy of the catalysts in general, ODB sensors can be used in engine control (for example a lambda probe, a NO.sub.x sensor, or an exhaust temperature sensor).
According to one preferred embodiment, a check is made during stratified operation of the internal combustion engine to determine whether throttling is sufficient to raise the temperature. If not, a switch is made in a second stage to homogeneous operation of the internal combustion engine.
In addition, the pre-ignition angle can be reduced to raise the temperature in the exhaust line. In order to react to factors which have an influence on the catalyst temperature, the exhaust temperature threshold value can be adapted as a function of the vehicle operating conditions or the vehicle ambient conditions. In particular, an adjustment can be made as a function of the ambient temperature and vehicle speed. By this measure, for example with the vehicle rolling (e.g. downhill), rapid cooling of the storage catalyst is prevented by early measures to raise the temperature. In addition, the temperature gradient could also be evaluated. When there is a sharp temperature drop in the catalyst, the engine can be switched directly to homogeneous operation.
All in all, the present method and apparatus according to the invention result in an appropriate temperature increase by which cooling of the catalyst is prevented, and good efficiency of an engine operated in non-throttled stratified operation is maintained.
Other objects, advantages and novel features of the present invention will become apparent from the following detailed description of the invention when considered in conjunction with the accompanying drawings.